In electrophotography, generally, a surface of a latent image bearing member is charged using a process such as corona discharge followed by exposure using laser to form an electrostatic latent image. The resulting electrostatic latent image is developed by a toner to form a toner image. An image with high quality can be obtained by transferring the resulting toner image on a recording medium. Typically, toner particles (toner base particles) with an average particle diameter of from 5 μm to 10 μm, produced by mixing a binder resin such as a thermoplastic resin with toner components such as a colorant, a charge control agent, a release agent, and a magnetic material and then passing the mixture through the steps of kneading, pulverizing, and classifying, are used for the toner applied to such electrophotography. The above-mentioned method of producing a toner, which includes mixing and kneading the materials in the toner, pulverizing the kneaded material, and classifying the pulverized material, is called a “pulverizing process”. In addition, in order to provide flowability or appropriate charging performance to the toner or to facilitate cleaning of the toner from surfaces of photoconductor drums, silica and/or inorganic fine particles such as those of titanium oxide are externally added to the toner base particles.
In regards to such a toner, for the purpose of improving low-temperature fixability, improving high-temperature storage stability, and improving blocking resistance, toner, which includes toner particles of a core-shell structure in which toner core particles using a binder resin of a lower melting point are coated with a shell material consisting of a resin with a glass transition point (Tg) higher than that of the binder resin in the toner core particles, have been used heretofore.
As for toner which includes toner particles with such a core-shell structure, a toner which includes toner particles with a core-shell structure, composed of toner core particles containing a polyester resin or a resin where a polyester resin and a vinyl resin are bound and a shell layer consisting of a shell material containing a copolymer between styrene and a (meth)acrylic monomer containing a polyalkylene oxide unit, has been proposed. The toner particles with this core-shell structure are formed by coating a surface of toner core particles with resin fine particles dispersed in an aqueous medium in the presence of an organic solvent such as ethyl acetate.
However, in cases of the toners produced using the pulverizing process, image defects called “void” or “letter scattering” tend to occur in the resulting images. Typically, toner particles in the toners obtained through the pulverizing process exhibit poor flowability because of an irregular shape with a low circularity. When forming images using a toner containing toner particles with a low circularity, a contact friction coefficient between the toner particles and a surface of latent image bearing member may increase and thus the toner particles may be resistant to be separated from the surface of latent image bearing member. In this case, image defects called “void” may occur in resulting images. Furthermore, in cases where a toner image on a surface of latent image bearing member is transferred on an intermediate transcriptional body such as an intermediate transfer belt and then the toner image on the intermediate transcriptional body is transferred on recording media to thereby form an image, if transfer failure has occurred, image defects called “letter scattering” (phenomenon in which a toner adheres near images such as letters in a condition that the toner is scattered in transferred images) tends to occur in resulting images.
Furthermore, in the shell layers of the toner particles in the toner, since contact sites of the resin fine particles themselves have been dissolved by the organic solvent, there remains almost no void between the resin fine particles and uniform films are formed in a condition that the shape of resin fine particles remains. Therefore, when forming images using the toner, the shell layer may be resistant to break during fixing images on recording media even when a pressure is applied to the toner particles in the toner. In cases where the shell layer cannot be easily broken, it is difficult to appropriately fix the toner on recording media.
In addition to the circumstances described above, in recent years, there is a circumstance that toner particles in toners have been small particle-sized on the ground of growing need for higher image quality. Thin-line reproducibility may be enhanced when images are formed by use of a toner containing toner particles with a smaller particle diameter.
However, the toner containing toner particles with a smaller particle diameter may exhibit lower developing ability. In many cases, the toner containing toner particles with a smaller particle diameter includes an ultrafine powder with a particle diameter of 3 μm or less. When the toner includes the ultrafine powder, the ultrafine powder tends to pollute development sleeve. The ultrafine powder has a strong adhesive force to development sleeve. Therefore, in cases of forming images using a toner containing a large amount of ultrafine powder, the ultrafine powder may stay in development sleeves rather than being supplied to a photoconductor when supplying the toner from the development sleeve to the photoconductor during development. If such a phenomenon is repeated, uneven distribution of the ultrafine powder strongly adhering to development sleeve occurs on the development sleeves (the uneven distribution is called “sleeve adhesion”). If the sleeve adhesion has occurred, developing ability tends to degrade since formation of thin layers of the toner may become insufficient on a peripheral surface of the development sleeves.